CMMG Inc Fastback Receiver Extension Mil-Spec Black. The Fastback Receiver Extension attaches to any firearm that accepts a Mil-Spec receiver extension (buffer tube). Once installed this allows you to easily pull the stock (or brace) back to an extended position without pressing a release button. Specifications for CMMG Inc Fastback Receiver Extension: Manufacturer: CMMG Color: Black Finish: Anodized Fabric/Material: Aluminium Condition: New Features of CMMG Inc Fastback Receiver Extension Fastback Receiver Extension 6 Position Mil-Spec Diameter Aluminum Black Anodized Finish P3 Screw Included Package Contents: CMMG Inc Fastback Receiver Extension Fastback Receiver Extension P3 Screw CMMG Inc 55CA93E: Fastback Receiver Extension Mil-Spec Black